Yep, that's exactly what must have happened, lot flooded. Here's some basic calcs

a 13.6 x 16 tire is 38.5inches in diameter and roughly 13.5 inches wide, taking out the 16 inch diameter for the rim, the volume is roughly 13,000 cubic inches, or 56 gallons, or in buoyant force, roughly 469 pounds of buoyant force per tire.

So, figure roughly 900 pounds of buoyant force on the back (given 2 rear tires) and a tractor that weighs just about 1,200 pounds, I'd say you end up with the tractor standing on it's nose! I'm just surprised that the water was slow moving and the tractors were steady enough to remain that way. If you look closely in the middle, there's another lo boy behind the others with the standard 9.3 x 24 tires on it, sitting normal. Very cool photo!

Tire fluid fill charts say 31 gallons for that size tire, 75% fill. That puts a full fill over 41 gallons, just for the air volume inside the tires. At 8+ pounds per gallon, you get 350# of buoyancy (31/.75x8.34). Times 2 for the pair gets you around 700#. To that, you add the volume of the tire material itself (another 15-20% ?) and all the volume of the rest of the back of the tractor (mainly 3 gear housings) and you are easily in a range where the back end would float, but well short of enough to float the whole tractor. If the front wheels hadn't remained on the ground, it is unlikely they would have remained lined up and spaced so evenly.